The invention relates to a hydrogenated amorphous carbon coating as well as a method for producing same. It also relates to devices comprising such a coating.
Frictional and wear phenomena in mechanical assemblies are the origin of considerable energy losses.
This is why many studies have been carried out over several years on coatings having high mechanical properties in terms of high wear resistance, hardness and toughness.
The coatings that are currently most encountered are coatings made of nitride, such as titanium nitrides (TiN) and chromium nitrides (CrN), carbides such as titanium carbides (TiC), chromium carbides (CrC), tungsten carbides (W2C) and tungsten/carbon carbides (WC/C), oxides such as alumina, deposits based on molybdenum such as molybdenum sulfide (MoS2), as well as all the family of coatings based on carbon called Diamond Like Carbon (DLC).
On account of their exceptional combination of properties, DLCs have revealed themselves to be excellent coatings for parts subject to high friction and wear problems, such as engine parts.
DLC coatings are carbon-based coatings. According to the preparative technique, they possess, in point of fact, not only high hardness but also low coefficients of friction and wear rates.
Some of these coatings are composed of a monolayer, but their main disadvantage is their high level of internal stresses, of the order of a few gigapascals (GPa), which very often limits their thickness to 2 micrometers, which is insufficient in some applications.
This is why research is currently turning toward the use of doped DLCs or of multilayer systems based on DLC.
In point of fact, multilayers based on DLC seem to be able to limit internal stresses and in this way to achieve very much higher coating thicknesses while preserving valuable mechanical properties.
Two main multilayer systems based on hydrogenated amorphous carbon, commonly called a-C:H, appear in the bibliography:
1) stacks of two a-C:H layers that differ from each other, one being said to be hard and the other said to be soft, as described by Gupta et al. in “Tribological behavior of plasma-enhanced CVD a-C:H films”, Tribology International 37, (2004), 1031-1038,
2) stacks of the a-C:H/a-SiC:H type.
However, these multilayer stacks, apart from the presence of different materials, have tribological and mechanical (hardness) properties lower than those of a-C:H monolayers.
The object of the invention is to overcome problems of the prior art by providing a multilayer coating based on hydrogenated amorphous carbon having mechanical properties, in terms of hardness, coefficient of friction and wear rate, equivalent to those of a monolayer or multilayer deposit of conventional hydrogenated amorphous carbon, but having lower residual stresses, which makes it possible to increase the total thickness of the coating to 10 micrometers.
To this end, the invention provides a hydrogenated amorphous carbon coating, characterized in that it comprises at least two layers of hydrogenated amorphous carbon having identical chemical compositions and physical and mechanical properties, and identical or different thicknesses.
Preferably, each of said hydrogenated carbon layers has a thickness less than or equal to 500 nm.
Preferably, the coating of the invention has a thickness greater than or equal to 1.5 micrometers, more preferably greater than or equal to 2 micrometers.
In a preferred embodiment, the coating of the invention comprises more than ten layers per micrometer of coating thickness.
The invention also provides a method for depositing the coating of the invention on at least one surface of a substrate comprising steps of depositing, by plasma-enhanced chemical vapor phase deposition, a first layer of hydrogenated amorphous carbon, for a period t1, of stopping the plasma generator for a period t, of depositing a second layer on the previously obtained layer by restarting the plasma generator under the same conditions of power, temperature, pressure and atmosphere as in the deposition step for the first layer, for a period t2 that is identical to or different from the period t1 of the deposition step for the first layer and, optionally, repeating the steps of stopping the generator and of depositing a new layer at last once until the desired coating thickness is obtained.
Preferably, the period t when the generator is stopped is less than the periods of the deposition steps.
Preferably, during the step when the generator is stopped, the upper surface of the layer obtained in the preceding step is scoured.
Preferably, this scouring is performed by ion bombardment of said upper surface in a neutral gas.
In the method of the invention, the periods t1, t2 of the deposition steps are such that the thickness of each layer deposited is less than or equal to 500 nm.
Preferably, in the method of the invention, the steps of stopping the generator and of depositing a new layer are repeated until a final coating thickness is obtained, preferably greater than or equal to 1.5 micrometers, more preferably greater than or equal to 2 micrometers.
Also preferably, the method of the invention additionally includes, before the step of depositing the first layer, a step of depositing an SiC:H layer on the surface of substrate to be coated.
The invention furthermore provides a device comprising a coating according to the invention or obtained by the method according to the invention.
The invention will be better understood and other advantages and features thereof will become more clearly apparent on reading the following description made with reference to the figures in which: